Lights Out for Dark Matter Claim?

Last November, data from a balloon-borne particle detector circling the South Pole revealed a dramatic excess of high-energy particles from space--a possible sign of dark matter, the mysterious substance whose gravity seems to hold our galaxy together. But satellite data reported today stick a pin in that claim. Researchers working with NASA's orbiting Fermi Gamma-ray Space Telescope say they do not see the purported excess. The observations don't disprove the existence of dark matter, but they put a damper on hopes that physicists had already begun to see it.

For decades, astrophysicists have known that galaxies don't contain enough ordinary matter to keep themselves from whirling apart. So they assume that some form of dark matter pervades each galaxy and provides the extra gravity needed to keep it whole. But physicists have never directly observed particles of dark matter, which are supposed to interact very weakly with ordinary matter.

One way to spot these particles might be to look to the skies. Some popular theoretical models suggest that if two lingering particles of dark matter collide, they should annihilate to create an ordinary particle and an antiparticle, such as an electron and a positron, which can be observed. Those particles should emerge with a definite energy determined by the mass of the dark energy particles, leading to a sharp peak in the energy spectrum of electrons and positrons from space.

That's why the results from the NASA-funded Advanced Thin Ionization Calorimeter (ATIC) balloon experiment sparked interest last fall (Science, 21 November 2008, p. 1173). ATIC observed that the number of electrons and positrons hitting Earth peaked sharply between about 300 billion and 800 billion electron volts. That dramatic excess appeared to be consistent with dark-matter annihilations.

But the new satellite measurements, from the $690 million Fermi telescope, don't reveal such an excess. Launched in June and designed to detect high-energy photons called gamma rays, Fermi is actually a sophisticated particle detector that serves just as well to detect electrons and positrons. It detected more than 4 million electrons and positrons from August through January--compared with ATIC's thousands--and Fermi researchers precisely measured the particles' energy spectrum.

The data do not reproduce ATIC's peak, Fermi researchers announced today at the annual April Meeting of the American Physical Society in Denver, Colorado. "We have much better statistics and can tell you that we do not see so extreme a feature" as was seen by the balloon-borne experiment, says Steven Ritz, a Fermi team member from NASA's Goddard Space Flight Center in Greenbelt, Maryland.

However, ATIC researchers aren't willing to concede that the Fermi result rules theirs out. Fermi has much poorer energy resolution, says John Wefel, an astrophysicist at Louisiana State University in Baton Rouge and leader of the ATIC team. That would turn any peak into a much less dramatic broad bump, and the Fermi spectrum does seem to show some sort of a gentle upwelling. "The difference comes down to something in the instrumentation," Wefel says.

Even if the Fermi results put the kibosh on hopes that dark matter has already been detected in a clean and simple way, there's still a chance physicists have seen subtler signs of it, says Neal Weiner, a theorist at New York University. He notes that the Italian PAMELA satellite last year reported an increase in the ratio of positrons to electrons at lower energies, which could hint at some manifestation of dark matter. And, as Wefel notes, the Fermi spectrum seems to include more high-energy electrons than expected, which could also be evidence of the mysterious substance.